When measuring microcurrent, it is necessary to primarily prevent leakage current from the pattern or the strip line on the measurement path, as well as leakage current from the relay on the measurement path. The reed relay cited in Japanese Patent (Kokai) 2001-14994 that uses a coil bobbin structure with which offset current attributed to heat-induced current and flowing to the reed relay is interrupted and that can be used for measurements on the order of femtoamperes (fA; 10−15 A) is a reed relay for microcurrent. The reed relay in FIG. 5 of JP (Kokai) 2001-14994 is shown in FIG. 6 of the present Specification. The structure of this reed relay will be described. A reed switch 501 is held inside electrostatic shield tube 503 by bushings 502a and 502b, which are made from a strong insulating material, and this electrostatic shield tube 503 is placed in the hollow cylinder part of coil bobbin 504. The area of contact between coil bobbin 504 and electrostatic shield tube 503 is such that space 509 is formed between the inside wall of coil bobbin 504 and the outside wall of electrostatic shield tube 503 by making projections 508a and 508b at the open ends of the hollow part of coil bobbin 504. Thus, joule heat that has been generated by coil 505 that is coiled around coil bobbin 504, virtually is not transmitted to electrostatic shield tube 503. As a result, it is possible to reduce the offset current that flows between relay terminals 517a and 517b and the respective bushings 502a and 502b, while, in the conventional type of the reed relays, the heat induced current is passed between bushings 502a and 502b and electrostatic shield tube 503 by transmitting joule heat to bushing 502a and 502b. 
Nevertheless, developments in measurement technology have led to a need for a reed relay with which the measurement of even smaller currents is possible.
Japanese Patent No. (Kokai) 8[1996]-279314 relating to dielectric absorption of bushings and Japanese Patent (Kokai) 2[1990]-68829 relating to leakage current and thermo-electromotive force are other technologies for reed relays for microcurrent.
The present invention provides a reed relay with which offset current in microcurrent measurement is further reduced. The present invention also provides a reed relay which reduces the offset current flowing between the reed switch and bushing that is attributed to heat-induced current blowing between the bushing and the electrostatic shield tube. Still the present invention provides a structure with which charge transfer from heat-induced current from the bushing to the electrostatic shield tube in a reed relay is canceled.